Non-dispersive Infrared (NDIR) techniques utilizing the characteristic absorption bands of gases in the infrared have long been considered as one of the best methods for gas measurement. These techniques take advantage of the fact that various gases exhibit substantial absorption at specific wavelengths in the infrared radiation spectrum. The term “non-dispersive” refers to the type of apparatus incorporating the NDIR technique, typically including a narrow band pass interference filter (as opposed to a “dispersive” element such as a prism or a diffraction grating to isolate and pass radiation in a particular wavelength band from a spectrally broad band infrared source. The gas concentration is discerned from the detected intensity modulation of source radiation emanating from the source and passed by the filter coincident in wavelength with a strong absorption band of the gas to be measured.
A prior art NDIR gas analyzer typically includes an infrared source with a motor-driven mechanical chopper to modulate the source so that synchronous detection is used to discriminate spurious infrared radiation from surroundings; a pump to push gas through a sample chamber; a narrow band-pass interference filter; a sensitive infrared detector, inexpensive infrared optics and windows to focus the infrared energy from the source onto the detector. Despite the fact that the NDIR gas measurement technique is one of the best methodologies that had ever been devised, it has not enjoyed wide application because of its complexity and high cost of implementation.
Several components play essential roles in making the NDIR technique work for gas measurement. The radiation source is one such component; and in order for NDIR methodology to work efficaciously, this source must provide broad spectral output and give high power in the band of interest. A blackbody radiation source is generally used for this component because it can be heated to a temperature that provides high intensity radiation within any wavelength region. Because the blackbody output spectral distribution and intensity are uniquely determined by the temperature of the source, the spectral peak intensity can be adjusted by varying the temperature of the blackbody source. By the well-known Wien's Displacement Law the peak intensity of a blackbody radiation source of temperature T is at a wavelength λmax equivalent to 2.898×10−3/T where T is measured in degrees Kelvin (1° K) and λmax is measured in meters. By the process of T and the size of the blackbody radiator, a desired intensity of blackbody radiation selected wavelength λ can be attained.
The so-called “Nernst Glower,” consisting of a heating element of a tungsten filament) embedded in a ceramic slab, has long been used for the blackbody source of most prior art NDIR gas detection systems. Despite the fact that the Nernst Glower gives off an amount of infrared energy with an emissivity close to unity, its power efficiency (i.e., efficiency from electrical to useable optical energy) is notoriously low. The large amount of unwanted heat is also a major drawback in the use of the Nernst Glower in any NDIR gas measurement systems. Furthermore, the large heat capacity of the Nernst Glower makes it a necessarily slow device in terms of intensity modulation. In many cases, it can only be used as a steady state or DC radiation source; and thus a mechanical chopper is needed to generate synchronous modulated signals, further adding to the complexity of NDIR gas measurement systems.
The prior art has attempted to replace the blackbody radiator with hot filament as an approximation to a blackbody. However, such a filament does not have high emission over all wavelengths because there is spatial temperature variation within the filament; it is not an ideal blackbody source. Optical radiation sources, such as a hot filament lamp, are thus often referred to as “quasi-blackbody” sources. In addition, hot filament radiators typically are used by a quartz bulb which is substantially opaque for wavelengths, longer than about 4.5 μm reducing its applicability to gas detection through longer wavelength absorption lines. Prior art hot filaments are thus not particularly good infrared radiation sources for use with NDIR gas sensors.
The prior art has attempted to improve NDIR sensors. In U.S. Pat. No. 4,876,413 (the '414 patent), published in April of 1975, Bridgham describes an infrared radiation source that includes a thin film resistor heater with highly emissive material Cr3Si on a substrate. The thin film heater is positioned between a pair of thin metal elements serving as sensing electrodes on a very thin (<0.005″ typical) insulating substrate. The entire thin film heater is packaged in a standard TO-5 heater equipped with a focusing reflector and pins supporting the heater structure.
While the source of the '413 patent advances infrared radiator sources in terms of higher emissivity and wider spectral emissions, it does not offer size or speed advance over the classic tungsten lamp. Furthermore, its construction is rather fragile and the heater cannot withstand temperatures above −700° C., severely limiting the maximum allowable output. Finally, its overall power efficiency is rather poor and the low cost and useable life of this radiator has not been satisfactorily proven. Consequently, the source of the '413 patent has not found wide application in NDIR gas measurement systems.
In U.S. Pat. No. 4,644,141 published in February of 1987, Hager et al. advances a heater structure first proposed in the '413 patent, except that a combination of silicon, silicon oxides and a platinum metal pattern are used as the heating element to optimize the performance of the overall heater structure. Nevertheless, other than a slight improvement in power efficiency over the device of '413 patent, there is no fundamental advancement over the prior art.
Over the past several years, significant technical progress has been made in the area of optical sources as set forth in the inventor's own prior application of U.S. Ser. No. 08/511,070, filed on Aug. 3, 1995. Such optical sources have greatly increased the reliability and cost-effectiveness of NDIR gas sensors.
The present invention has several objects in providing further improvements and advantages in source and sensor embodiments as compared to the prior art. One object of the invention is to provide sources which function as tuned waveband emitters that preferentially emit radiation into a wavelength band of interest as compared to blackbody or gray-body radiators. Another object of the invention provides in integrated circuit sensor include selectively tuned radiation source. Still another object of the invention provides methods and devices for sensing gas constituents without certain of the difficulties and problems of prior art NDIR devices.
These and other objects will become apparent in the description which follows.